The present invention relates to a speaker unit, and more particularly to a loudspeaker unit for a type of loudspeakers called a dome speaker, wherein a diaphragm is accurately positioned with respect to a speaker frame.
There is known a speaker unit for a dome loudspeaker.
Referring to FIG. 2, a conventional dome speaker has a cylindrical yoke 1 having an upper opening, and an annular speaker frame 2 attached to the yoke 1 on the upper periphery thereof. A magnet 3 such as a permanent magnet is securely mounted in the yoke 1 and a diaphragm 4 is mounted over the speaker frame 2.
The diaphragm 4 comprises a central spherical vibrating portion 5, inner cylindrical portion 6 connected to the base of the spherical portion 5, an outer cylindrical portion 7 formed by upwardly turning the bottom edge of the inner cylindrical portion 6, and an outwardly extending support portion 8 bent at the upper edge of the outer cylindrical portion 7. A voice coil 9 is mounted in an annular gap between cylindrical portions 6 and 7, while maintaining a gap 10 between the coil 10 and the inner periphery of the outer cylindrical portion 7. The supporting portion 8 is securely attached to the speaker frame 2. The diaphragm 4 is formed by molding a resin film with a press. The portion between the outer cylindrical portion 7 and the supporting portion 8 is upwardly bulged to form a semicircle in section.
The speaker frame 2 made of a thermoplastic synthetic resin has an integral annular shoulder 11 formed on the inner periphery thereof. The supporting portion 8 of the diaphragm 4 is securely attached to the shoulder 11 with an adhesive so that the diaphragm 4 is securely mounted on the frame 2.
When excited, the voice coil 9 generates an electromagnetic force in the axial direction thereof, that is, in the upward direction toward the upper edge of the sheet of the figure, thereby vibrating the diaphragm 4. Hence sound is emanated from the spherical portion 5 of the diaphragm 4. Although speaker units for dome speakers which vary in detail are known, each has the same basic construction as described above.
When assembling the speaker unit, it is essential that the center of the spherical portion 5, that is the center of the inner cylindrical portion 6, coincides with the center of the speaker frame 2, that is the center of a magnetic circuit. When the centers do not coincide with each other, the inside wall of the cylindrical portion 6 contacts with the magnet 3 because of rolling of the diaphragm 4, thereby emanating mechanical noises other than electrical signal sounds, such as scratch noise, causing a defect in the product. Hence it is necessary to accurately position the diaphragm 4 in the speaker unit at assembling.
Referring to FIG. 3, in a conventional assembling method, when mounting the diaphragm 4 on the speaker frame 2, the outer edge of the supporting portion 8 is made to slide on a vertical inner wall 12 which is extended from the shoulder 11. More particularly, the diaphragm 4 is formed with a mold and then the outer periphery thereof is cut out with a die to form an entire edge, so that the diameter of the supporting portion 8 conforms to the inner diameter of the vertical wall 12. Thus by sliding the outer edge of the supporting portion 8 along the vertical wall 12, the diaphragm 4 can be positioned.
However, the positioning by the conventional method may not be sufficiently accurate. Namely, since the molding and the cutting out of the diaphragm 4 are made in different processes, a center O5 of the spherical portion 5 and a center O8 of the supporting portion 8 may be deflected from one another as shown in FIG. 4. Such an deflection causes the inner wall of the inner cylindrical portion 6 to contact with the magnet 3, emitting the above described noises.
In order to solve the problem, there is proposed a structure shown in FIG. 5, which is actually put into use.
Referring to FIG. 5, an annular rib 13 is provided on the supporting portion 8 to form an engaging recess on the underside thereof. The shoulder 11 of the speaker frame 2 has an annular projection 14 which is adapted to engage with the recess of the rib 13. Since the rib 13 is formed at the same time with the molding of the diaphragm 4, the rib 13 is shaped coaxially with the diaphragm. Thus, when the rib 13 engages with the projection 14, the diaphragm 4 is accurately positioned so that the center of the diaphragm coincides with the center of the voice coil 9 around the inner cylindrical portion 6.
In such a structure, since it is necessary to form the rib 13 on the supporting portion 8 and the projection 14 on the shoulder 11, the widths in the radial direction thereof are increased, thereby preventing reduction of the size of the loudspeaker as a whole. The materials for the diaphragm 4 and the speaker frame 2 are also increased, so that the structure is undesirable from the point of the manufacturing cost.